CVE-2020-26265

### Impact A consensus-vulnerability in Geth could cause a chain split, where vulnerable versions refuse to accept the canonical chain. ### Description A flaw was repoted at 2020-08-11 by John Youngseok Yang (Software Platform Lab), where a particular sequence of transactions could cause a consensus failure. - Tx 1: - `sender` invokes `caller`. - `caller` invokes `0xaa`. `0xaa` has 3 wei, does a self-destruct-to-self - `caller` does a `1 wei` -call to `0xaa`, who thereby has 1 wei (the code in `0xaa` still executed, since the tx is still ongoing, but doesn't redo the selfdestruct, it takes a different path if callvalue is non-zero) - Tx 2: - `sender` does a 5-wei call to 0xaa. No exec (since no code). In geth, the result would be that `0xaa` had `6 wei`, whereas OE reported (correctly) `5` wei. Furthermore, in geth, if the second tx was not executed, the `0xaa` would be destructed, resulting in `0 wei`. Thus obviously wrong. It was determined that the root cause was this [commit](https://github.com/ethereum/go-ethereum/commit/223b950944f494a5b4e0957fd9f92c48b09037ad) from [this PR](https://github.com/ethereum/go-ethereum/pull/19953). The semantics of `createObject` was subtly changd, into returning a non-nil object (with `deleted=true`) where it previously did not if the account had been destructed. This return value caused the new object to inherit the old `balance`: ```golang func (s *StateDB) CreateAccount(addr common.Address) { newObj, prev := s.createObject(addr) if prev != nil { newObj.setBalance(prev.data.Balance) } } ``` It was determined that the minimal possible correct fix was ```diff +++ b/core/state/statedb.go @@ -589,7 +589,10 @@ func (s *StateDB) createObject(addr common.Address) (newobj, prev *stateObject) s.journal.append(resetObjectChange{prev: prev, prevdestruct: prevdestruct}) } s.setStateObject(newobj) - return newobj, prev + if prev != nil && !prev.deleted { + return newobj, prev + } + return newobj, nil ``` ### Patches See above. The fix was included in Geth `v1.9.20` "Paragade". ### Credits The bug was found by @johnyangk and reported via [email protected]. ### For more information If you have any questions or comments about this advisory: * Open an issue in [go-ethereum](https://github.com/ethereum/go-ethereum) * Email us at [[email protected]](mailto:[email protected])